This invention relates to a sensor system for measuring linear displacement of a first member relative to a second member, comprising an analogue Hall Effect sensor secured to the first member in an orientation to sense magnetic flux in a sensing direction perpendicular to said direction of a relative movement, and a permanent magnet secured to the second member and having a front surface facing the Hall Effect sensor and extending along the direction of relative movement between the first and second members.
An analogue Hall Effect sensor produces an output voltage related to the component of the flux density of a magnetic field in which it is located which is perpendicular to its sensing surface. The sensor produces zero output voltage when subject to a magnetic field of sufficient strength in one direction and its maximum output voltage when subject to a magnetic field of the same magnitude in the opposite direction. In the absence of a magnetic field, the sensor produces an output voltage of half its maximum voltage.
GB-A-1109220 disclosed a sensor system of this type in which the front surface is concave in the direction of relative movement so that the magnetic flux at the Hall Effect sensor has a minimum value when the latter is positioned opposite a central location at which the concavity of the surface has maximum depth. A second Hall Effect sensor is mounted adjacent to the first sensor for simultaneous movement therewith. The outputs of the two sensors are combined electronically in order to determine the side of the central location on which the sensors are located.